Speculation

Contradiction in Terminology

Parson states at one point that "any unit with leadership ability is called a Commander or Warlord" and "these units give a bonus to units under their command"Erf-b1-p040a.

That contradicts with other statements that "only warlords have Leadership" Erf-b1-p084a and that Casters are Commanders tooErf-b1-p084a, but with no Leadership ability.

It seems that "Commander" is a more general term - Commanders have an ability to lead other units in combat (not necessarily deriving from Leadership natural ability) while Warlords are Commanders who have Leadership ability and thus give units under their command a Leadership bonus. It should be noted that in some special cases Casters can give a bonus to certain units under their command (known examples are Golems led by a Dirtamancer and uncroaked led by a Croakamancer)Erf-b1-p084a.

Therefore, every Warlord is a Commander, but not every Commander is a Warlord.

Duty

Duty affects Commanders only. Has Higher effect on Warlords, highest on Chief Warlord. Duty requires Commanders to use their own initiative in the service of the Ruler. Another effect of Duty is that Commanders cannot withhold information and cannot conspire against the Ruler.Erf-b1-p084a

Use as a Title

Word "Commander" is used as a title to address Warlords and, occasionally, Casters. In this sense words "Warlord" and "Commander" seem to be used interchangeably, although Commander seems to express more respect.